Structure-Property Relationships in Unsymmetric Bis(antiaromatics): Who Wins the Battle Between Pentalene and Benzocyclobutadiene?

According to the currently accepted structure-property relationships, aceno-pentalenes with angular shape (fused to the 1,2-bond of the acene) exhibit higher antiaromaticity than those with linear shape (fused to the 2,3-bond of the acene). To explore and expand the current view, we designed and synthesized molecules where two isomeric, yet, different, 8π antiaromatic subunits, a benzocyclobutadiene (BCB) and a pentalene are combined into, respectively, an angular and a linear topology via an unsaturated 6-membered ring. The antiaromatic character of the molecules are supported experimentally by 1H NMR, UV-Vis and cyclic voltammetry measurements and X-ray crystallography. The experimental results are further confirmed by theoretical studies including the calculation of several aromaticity indices (NICS, ACID, HOMA, FLU, MCI). In the case of the angular molecule, double bond-localization within the connecting 6-membered ring resulted in reduced antiaromaticity of both the BCB and pentalene subunits, while the linear structure provided a competitive situation for the two unequal [4n]π subunits. We found that in the latter case pentalene drew the shorter straw. The BCB unit alleviated its unfavorable antiaromaticity more efficiently, leaving the pentalene with enhanced antiaromaticity. Thus, a reversed structure-antiaromaticity relationship when compared to aceno-pentalenes was achieved.